Aged felt garage roof with blisters, cracking and moss along the seams, viewed from a ladder

Is your felt roof on the way out?

The five signs a felt roof is failing, what actually causes it, and whether to patch it, re-felt it or upgrade to rubber. Costs updated July 2026.

Quick answer: a felt roof showing blisters, cracks or soft spots is usually near the end of its life. A patch repair costs £100 to £300 in 2026, and a full re-felt runs £80 to £110 per m², which puts a typical single garage at £1,400 to £2,000. Full prices are in our felt roof cost guide.

Felt is the cheapest flat roof covering and the shortest lived, so most garages, extensions and porches will see a felt roof fail at some point. The good news: it rarely fails without warning. Here is what to look for, and what each sign is telling you.

The five signs a felt roof is failing

Blistering: raised bubbles where trapped moisture or air expands in the sun. Isolated blisters can be patched; widespread blistering means moisture is through the covering.
Alligator cracking: a web of fine cracks across the surface, named after the skin pattern it makes. It means the bitumen has oxidised and gone brittle. This does not repair well.
Bald mineral patches: the green or grey mineral surface has worn away, exposing the black bitumen beneath to UV. Bald felt ages fast.
A soft, spongy deck: if the roof gives underfoot or under hand pressure, water has reached the timber boards below. This is the most serious sign on the list.
Moss in the seams: moss needs sustained moisture to grow, so a mossy lap joint is telling you that joint no longer sheds water properly.

Why felt roofs fail

Ranked by how often we see each one: age and UV come first, because bitumen simply dries out and cracks after years of sun. Trapped moisture is next, blistering the covering from beneath, often because the felt was laid on a damp deck. Then thermal movement, which opens lap joints as the roof expands and contracts daily; ponding water, which sits on low spots and works into every weakness; and finally foot traffic and impact, from ladders, aerial fitters and dropped branches.

What to do right now

If water is already coming through, move anything valuable out from under the roof and get a temporary patch or tarp on quickly; a flat roof repair visit can make it watertight while you decide on the proper fix. If the roof is failing but dry inside, you have time to plan, so avoid walking on it (a brittle roof cracks underfoot) and get it looked at before winter rather than after.

Repair, re-felt or upgrade?

Repair makes sense for one-off damage on a roof that is otherwise sound: a single split, one blister, a lifted lap. Re-felting is the right call when the surface shows widespread cracking or blistering, because the covering itself has failed and patches just chase the leak around. Upgrading to EPDM rubber deserves a serious look whenever you are stripping the roof anyway: a single seamless sheet with no joints to fail, and usually the cheapest option per year of life even where the upfront cost is higher. Our EPDM guide compares the two properly, and the felt roof cost guide has the full 2026 price breakdown for each route.

Worth knowing: if the deck feels soft anywhere, insist on a strip-and-inspect rather than an overlay. New felt over rotten boards looks fine for a year and then fails with the deck, and you pay for the job twice.

How to make the next roof last longer

Keep gutters and outlets clear so water never ponds on the roof.
Sweep off leaves and moss once or twice a year; do not jet wash felt.
Use walkway boards if the roof needs regular access.
Have laps and flashings checked after big storms, while problems are still cheap.

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Felt roof FAQs

Failing felt roof questions, answered

Most traditional felt flat roofs last somewhere between 10 and 20 years, depending on the quality of the original installation, how much sun the roof takes and whether water ponds on it. High-performance torch-on systems sit at the top of that range; cheap shed-grade felt can fail much sooner.

One or two isolated blisters on an otherwise sound roof can usually be cut, dried and patched for £100 to £300. Widespread blistering is different: it means moisture is trapped through the covering, and patching one blister just moves the problem along. At that point re-felting is the honest recommendation.

Sometimes a single overlay is acceptable if the existing layer is dry, flat and well bonded, but it hides the deck, traps any existing moisture and adds weight. If the deck feels soft or spongy anywhere, overlaying is the wrong call: the old covering should be stripped so the boards underneath can be inspected and replaced.

Re-felting costs £80 to £110 per square metre in 2026, which puts a typical single garage at £1,400 to £2,000. Patch repairs run £100 to £300. If boards under the felt have rotted, replacing them adds to the bill, which is why quotes are usually confirmed after the strip. See the full felt roof cost guide.

For most domestic flat roofs, yes. EPDM is a single sheet with no seams to fail, is not vulnerable to blistering in the same way, and typically outlasts felt by a wide margin, so it usually works out cheaper per year of life even where the upfront price is higher. Our EPDM guide has the full comparison.